It has heretofore been known that titanium based catalyst composed of a titanium compound and an organoaluminum compound, or vanadium based catalyst composed of a vanadium compound and an organoaluminum compound are used as catalyst for the preparation of .alpha.-olefin polymers, for example, ethylene polymers or ethylene/.alpha.-olefin copolymers.
On the one hand, there have been proposed recently processes for the preparation of ethylene/.alpha.-olefin copolymers using catalysts composed of zirconium compounds and aluminoxanes as a new type of Ziegler catalysts for olefin polymerization.
In comparison with known catalyst systems composed of transition metal compounds and organoaluminum compounds, the catalysts composed of such transition metal compounds and aluminoxanes as proposed in the prior art as mentioned above are markedly excellent in polymerization activity. However, most of the catalyst systems as proposed above are soluble in the reaction system where they are used, and often used in the solution polymerization system, with the result that the process for the polymerization of olefins to which they are applied is limited. In addition thereto, when high molecular weight polymers are intended to obtain, the viscosity of the solution in the polymerization where said catalyst systems are used becomes markedly, and the polymers resulting from the post-treatment of the solution polymerization system are found to be low in bulking density. In view of the foregoing, it was difficult to obtain polymers excellent in particle properties by the use of such proposed catalyst systems.
On the one hand, there have been made attempts to carry out polymerization of olefins in the suspension or vapor phase polymerization system using catalysts comprising a porous inorganic oxide carrier, such as silica, silica alumina or alumina, having supported thereon either one or both of the above-mentioned transition metal compound and aluminoxane.
For example, Japanese Patent L-O-P Publns. Nos. 35006/1985, 35007/1985 and 35008/1985 disclose that catalysts comprising a transition metal compound and aluminoxane supported on silica, silica alumina or alumina may be used in the above-mentioned attempts.
Japanese Patent L-O-P Publn. No. 276805/1986 proposes a process for the polymerization of olefins carried out in the presence of a catalyst comprising a zirconium compound and a reaction mixture obtained by reacting a reaction mixture of an aluminoxane and trialkylaluminum with an inorganic oxide containing a surface hydroxyl group such as silica.
Japanese Patent L-O-P Publns. Nos. 108610/1986 and 296008/1986 propose a process for the polymerization of olefins carried out in the presence of a catalyst comprising a transition metal such as metallocene and an aluminoxane supported on a support such as an inorganic oxide.
In the processes proposed in the prior art literatures as mentioned above, however, the aluminoxane component used in the catalyst had to be synthesized separately.
Japanese Patent L-O-P Publn. No. 31404/1986 proposes a process for the polymerization or copolymerization of ethylene or ethylene and .alpha.-olefin carried out in the presence of a mixed catalyst comprising a product obtained by reaction of trialkylaluminum with water in the presence of silicon dioxide oz aluminum oxide and a transition metal. According to this process, a separate step to synthesize aluminoxane can be omitted. In this process, however, the bulking density of the polymer obtained was as low as 0.2 g/cm.sup.3.
Furthermore, Japanese Patent L-O-P Publn. No. 207303/1989 teaches a process for obtaining a vapor phase polymerization catalyst by adding a metallocene to a reaction mixture obtained by reaction of green silica gel with trialkylaluminum in a solvent, followed by removing the solvent from the resulting mixture and then by drying. This process, however, requires additionally many steps necessary for the synthesis of catalyst such as solvent removing step, drying step, etc., though a separate step for the synthesis of aluminoxane can be omitted.